1. Field of the Invention
This invention relates to the field of transducers for partially or fully implantable hearing aids for direct mechanical excitation of the middle or inner ear. More specifically, this invention relates to such transducers including a housing which can be fixed at the implantation site with respect to the skull and a coupling element which can move with respect to the housing, the housing accommodating an electromechanical transducer by which the coupling element can transmit vibrations from the electromechanical transducer to the middle ear ossicle or directly to the inner ear.
2. Description of Related Art
A transducer arrangement of this general type is known from U.S. Pat. No. 5,277,694. In this reference, it is proposed that one wall of a housing be made as a vibrating membrane with an electromechanically active heteromorphic composite element with a piezoelectric ceramic disk attached to the side of the membrane inside the housing. Generally, a hearing aid transducer built in this manner can be implanted without difficulty and generally good results have been achieved. However, it has been found that at low frequencies, the coupling element driven by the piezoelectric ceramic disk does not create sufficient deflections to provide adequate loudness level for patients with medium and more serious hearing loss. This insufficient deflection has been attributed, in part, to be caused by the low electrical voltages required for such implants.
U.S. Pat. No. 5,624,376 discloses a transducer for partially or fully implantable hearing aids based on the electromagnetic principle in which a permanent magnet, together with two assigned pole pieces, is loosely suspended in a cylindrical housing. An induction coil which interacts with the permanent magnet is positioned in a cylindrical air gap bounded by the pole pieces and is permanently joined to one housing wall. When an AC voltage is applied to the coil, a grounded portion consisting of the permanent magnet and the pole pieces is vibrated. This grounded portion is mechanically coupled to the housing which is designed for attachment to a vibratory structure of the ear. Thus, when the grounded portion is vibrated, the resulting vibration of the housing stimulates the vibratory structure of the ear. However, it has been found that surgical implantation of such transducers is very difficult. Providing support for the housing and also mechanically coupling the housing with the vibratory structure of the ear (such as the ossicular chain) has been found to be especially difficult to surgically implement.
In an article by Fredrickson et al. entitled "Ongoing Investigations Into An Implantable Electromagnetic Hearing Aid for Moderate to Severe Sensorineural Loss", Otolaryngolic Clinics of North America, Vol. 28, No. 1, (February 1995), the authors mention an implantable middle ear transducer which drives a biocompatible probe tip and is accommodated in a hermetically sealed housing made of stainless steel. The probe tip fits into an opening on the incus which is formed by a laser beam. This reference, however, fails to disclose how the electromagnetic transducer is made or operated or how vibrations are transmitted from the electromagnetic transducer to the probe tip.
Therefore, there exists an unfulfilled need for electromagnetic transducers for partially or fully implantable hearing aids which provide direct mechanical excitation of the middle or inner ear while avoiding the surgical difficulties of the prior art transducers.